Most of approximately 540 land-based nuclear power reactors in operation throughout the world are pressurized water reactors (PWR) and the remainder for the most part, are boiling water reactors (BWR). The main exception is in the USSR where large graphite moderated water cooled reactors are used. In Canada and a few other places in the world heavy water reactors are used and may be gas cooled as used in England. In 1979 a PWR reactor at Three-Mile Island underwent a reactor core melt down, a financially disastrous event that did not entail any human injuries. This event triggered a search for emergency core cooling systems or for a reactor design that would never encounter a core melt down under any condition of equipment failure, control failure, or human error. The resulting effort produced many designs that involve valves, pumps, and controls but since these require unfailing electrical power the designs have not been fully satisfactory. It is believed that there are only two designs which have any real likelihood of success. One is the liquid metal cooled reactor at Idaho Falls owned by the U.S. Government. This system is dependent on the liquid sodium metal always being present in the core under all accident or failure conditions. Obviously, this cannot be guaranteed, since reactor and the surrounding back-up vessel having liquid sodium inside could both fail. Another danger of this system is that sodium reacts violently with water and this could happen if there were any leaks in the steam generator. Furthermore, there has developed a strong preference for water cooled reactors, and there is much resistance to changing to sodium.
The other design that appears to have the possibility of success is the pebble bed reactor now under development in Germany. This system uses helium as a coolant and graphite balls about 6 cm in diameter infused with fissile or breeder material. This design has many good features and may be accepted by utilities in due time. The main criticism of it is that it employs helium as a coolant which must be conducted through steam generators to make steam for use in a turbine, or the helium can be conducted directly into a gas turbine.
It is an object of the present invention to provide a novel system for utilizing a nuclear fission reaction to produce superheated vapor, e.g., steam, and will automatically reduce its energy output in case of a malfunction so as to eliminate the possibility of a melt down. It is still another object of this invention to provide a novel system employing pelletized fuel in a boiling liquid reactor which produces superheated vapor. Still other objects will become apparent from the more detailed description which follows.